The present invention generally relates to a telecommunication system in which ATM cells are being used for transmission of both voice-type data and other types of user data. The voice-type data is carried in microcells which in turn are carried in the payload of the ATM cells. The other types of user data are carried directly in the payload of the ATM cells.
More specifically, the invention relates to a telecommunication system comprising a sending entity, a receiving entity and an ATM link between the sending and receiving entities for transferring ATM cells on specific separate ATM connections. The ATM cells include user ATM cells having a header and payload containing data in the form of a determined number of data units which carry data in microcells, the header including an ATM connection pointer. The microcells have payload for carrying a variable size user data package and a microcell header containing code (CID) identifying a microcell connection. A microcell being not able to fit in its entirety in a remaining space of an ATM cell belonging to a certain ATM connection is divided into a first part located in at least one following ATM cell belonging to the same ATM connection. Microcell size information is associated with each microcell.
The method according to the invention relates to transferring microcells in a-telecommunication system of the kind just described.
Asynchronous Transfer Mode (ATM) is a technology that provides for efficient utilization of bandwidth in a telecommunications network for a broad variety of telecommunication services. Due to the large capital in existing equipment based on Synchronous Transfer Mode (STM) technology though, a mixed network type in which both ATM and STM technology resides is likely to evolve and persist for quite some time.
ATM stipulates that data be carried in fixed-size packets of 53 octets. The packets are referred to as ATM cells. The ATM cell size however penalizes some narrowband services. User information relating to such a narrowband service, e.g. voice samples of the Plain Old Telephony Service (POTS), is converted in a user terminal, e.g. a telephone, or elsewhere into a low-rate stream of digital data. This data is referred to as voice-type data. Voice-type data may originate from other information sources than voice, such as compressed video.
A time required to assemble data and place it in the payload of a cell causes a delay referred to as a cell assembly delay. Assembling enough voice-type data from a user terminal to completely fill an ATM cell takes considerable time. At a rate of 64 kbit/s, typical to POTS, awaiting 48 octets which can be accommodated in the payload of an ATM cell takes 6 ms. The cell assembly delay is hence 6 ms.
Each time voice-type data in the previously said low-rate stream format enters an ATM domain of the aforementioned mixed network, a 6 ms cell assembly delay is appended to the total delay of the voice-type data.
Voice-type data is delay sensitive, typically as it relates to interactive services. Network deficiencies necessitate echo cancellers when the total delay, including e.g. propagation delay, exceeds a certain value. This value depends on the network quality, and is sometimes less than 25 ms. Echo cancellers add to the cost for providing the service. Above a 100 ms delay, a service degradation is starting to become evident to a user, even with echo cancellers.
With a 6 ms cell assembly delay, a total voice data delay budget for attaining a high service quality is easily overdrawn. The service quality may not even be acceptable to the user. New voice services, such as mobile telephony, employ less bandwidth than do traditional POTS:s. The cell assembly delay for such lower bandwidth services is larger than for the POTS, thereby even further aggravating the situation.
ATM cells can optionally be only partially filled with voice data, resulting in a lower cell assembly delay. The bandwidth efficiency of partially filled ATM cells is however not as high as the efficiency of completely filled cells. An ATM cell carrying e.g. 4 voice data octets has a bandwidth efficiency of less than 10%.
A reasonable delay of voice-type data can be accomplished without sacrificing a lot of bandwidth in the ATM domains, by carrying voice-type data in variable-size packets in the payload of ATM cells. Such packets are referred to as microcells, as they are typically considerably smaller than an ATM cell, however similar in structure and usage.
Several microcells can be accommodated in the same ATM cell. Utilization is further improved by allowing a microcell which can not be fit in its entirety in the remaining space of an ATM cell, to be divided into a first part filling the remaining space of one such ATM cell, and a second part to be placed in another ATM cell.
A problem however, in a node for receiving microcells, is finding the boundaries of the microcells, such that microcell synchronization can be achieved.
In GB 2,270,820-A a STM/ATM network interface is described, in which information is carried in packets, which in turn can be carried in ATM cells. Parity and a packet-size indicator in the packet header is used for synchronization.